This invention relates generally to cooking appliances, and more particularly to relay-controlled cooking appliances, especially those which utilize electrical heating elements. More particularly, the invention relates to fail-safe electrical control systems for cooking appliances, and especially to those utilizing microcomputers as the controller, and which are adapted to ensure that the cooking appliance will not fail in a "runaway" mode that prevents the user from switching off the heating elements.
An ever-present danger associated with cooking appliances is a "runaway" failure, in which the control fails in a manner that prevents the user from turning off the heat source. This potential is particularly threatening with a cooking range having surface units because such surface units are exposed and, therefore, present a more imminent prospect of combustion or injury. Electronic controls for cooking appliances provide a significantly larger number of components than conventional electro-mechanical controls and present a greater number of failure modes. Microcomputers incorporated into such electronic controls are also subject to failure and typically fail with all output ports locked in one particular state.
While fail-safe controls have been proposed heretofore, prior attempts have had limited success. One such fail-safe control proposed in U.S. Pat. No. 4,719,326 to Yoo is intended to prevent power from being applied to the power relay of a cooking apparatus if a number of output ports of its controlling microcomputer fail in a common state. One problem with this type of control is that the interface circuitry occupies several output ports of the microcomputer, which are thus unavailable for other control functions other proposed fail-safe controls do not protect against single-component failure in the circuitry interfacing the microcomputer and the power relay.